One of the prominent themes explored throughout The Giver concerns conformity versus individuality. In Jonas's highly structured, austere community, individuality is virtually nonexistent and citizens are required to conform to society's standards or risk being released. The community is also founded on the principles of Sameness, which significantly limit personal freedoms and dramatically alter the natural environment. The Committee of Elders is the ruling political body of the community and the committee decides each person's occupation, matches spouses, creates family units, and even regulates the birthrate. The primary source of Jonas's anxiety at the beginning of the story concerns his lack of control in choosing his future occupation.

Citizens' thoughts and ideas are even suppressed and controlled through the use of language. The committee requires each person to use precise language, which severely limits their independent views and opinions. At a young age, children are taught to use precise language and censor their thoughts. During Jonas's training sessions as the community's Receiver of Memory, he learns the importance of individuality, personal choice, and independence. Jonas is fascinated by colors and expresses his desire to experience personal choice in chapter 13 by telling the Giver that people should have the right to choose the color of their clothing. As Jonas becomes more enlightened, he begins to challenge conformity by refusing to take his pills for the Stirrings and transferring memories to Gabriel. After discovering the truth behind the term "release," Jonas challenges conformity and Sameness by formulating a plan with the Giver to completely transform their oppressive community.

The Giver has numerous themes. Based on the question, it appears that one theme should be picked and supported. The previous answer did a nice job highlighting the importance of choice in the novel. I think an equally important theme is the importance of memory.

While the society within the novel has adopted the Sameness and just about everybody no longer remembers the past, the fact that they have a Receiver of Memory indicates that deep down they know that those past memories are important to keep. The story explains that the memory keeper is there to remind the elders of past mistakes. If they didn't have a way of storing those memories, the society would be doomed to possibly repeat past mistakes.

Tied with memory are emotions. The people in the society don't remember what pain and emotional hurt feels like; therefore, it essentially doesn't exist in the present. As Jonas learns from his time with the Giver, those memories of pain and hurt are valuable and important. Jonas realizes that in order to know what true happiness and joy feels like, he has to have something to compare it to. The painful memories are what allow him to know what happiness is truly like.

Jonas eventually comes to believe that the society would be better off having their memories back. He and the Giver discuss the possibility.

“If you floated down the river, I suppose I could help the whole community the way I’ve helped you. It’s an interesting concept. I need to think about it some more."

Jonas will eventually leave in order to save Gabriel from being released, but readers have to believe that Jonas and the Giver would have figured out a way to get the memories safely back to the people. This is why the plan was for Jonas to leave and the Giver to stay behind. They care about the people's ability to handle the returning memories.

Of course they needed to care. It was the meaning of everything.

The main theme of The Giver is that differences should be celebrated instead of feared.  Choice is instrumental to our humanity.

An example is when Jonas is given the memory of grandparents, and experiences love for the first time.

"I will. I am left with a vague wisp of that one; and I have many other memories of families, and holidays, and happiness. Of love." (p. 125)

Jonas comments that it is an impractical way to live, but he enjoys the experience of love.  Love is absent from the community because people may not make the best choices, and might experience pain.  However, the pain is worth it because where there is love, there is also joy.

A second example is when Jonas’s father releases the newborn twin.  Before Jonas sees the release, he comments on why the identical twins are considered unnecessary in the community.

"Well, they can't have two identical people around! Think how confusing it would be!" Jonas chuckled. (p. 146)

The community has placed the threat of confusion as more important than the life of the twin.  After Jonas sees what release really is, he is horrified.  He realizes his mistake, and his community’s mistake.

The final example is that the community tries to avoid having people make their own choices, in case they have to make the wrong ones.

"Definitely not safe," Jonas said with certainty. "What if they were allowed to choose their own mate? And chose wrong?

"Or what if," he went on, almost laughing at the absurdity, "they chose their own jobs?"

"Frightening, isn't it?" The Giver said. Jonas chuckled. "Very frightening. I can't even imagine it. We really have to protect people from wrong choices." (p. 98)

This relates to the theme because the community has chosen to eliminate choice, and the result is that no one is significantly different than anyone else.